In this specification the term “light” will be used in the sense that it is used in optical systems to mean not just visible light, but also electromagnetic radiation having a wavelength outside that of the visible range.
Wideband fibre-optic networks use wavelength division multiplexing to operate across a wide wavelength range and a large number of channels, e.g. approximately 100 channels that are arranged at 50 GHz spacings. Standard wavelength ranges defined by the International Telecommunications Union (ITU) for telecommunications include the C and L wavelength bands, which are 191.6-196.2 THz (approximately 1530 to 1565 nm) and 186.4-191.6 THz (approximately 1565 to 1610 nm) respectively.
Optical amplifiers are used to amplify optical signals that have become attenuated through transmission across such fibre-optic links of such networks. Raman amplification is a technique in which high power light is injected into a host material (e.g. an existing span of optical fibre), providing optical gain to optical signals passing through the host material via a stimulated Raman scattering (SRS) process. In optical fibre communications, pump lasers have been used to provide Raman amplification in the optical fibre spans of links.
Pump lasers for Raman amplification typically operate at shorter wavelengths than the wavelength of the optical data signal that requires amplification, e.g. for signals in the ITU C-band, the pump laser typically operates in the range 1440 to 1480 nm. Raman amplification systems may be used independently or alongside other optical amplification systems, such as erbium doped fibre amplifiers (EDFAs).
It is commonly required to provide Raman amplification with an amplification spectrum across the range of operating channels of a fibre-optic link that is tailored to the link, or which is uniform.
Distributed feedback (DFB) pump lasers are used as pump lasers for Raman amplification systems. DFB lasers have a relatively fixed, peaked optical intensity output spectrum, and produce a fixed peaked Raman amplification spectrum. The output spectrum and amplification spectrum of the DFB laser may be thermally tuned over a limited range of no more than a small wavelength range (up to 5 nm).
It is also known to use distributed Bragg reflector (DBR) type pump lasers comprising a gain section and a distributed Bragg reflector having a constant pitch grating. Such DBR pump lasers also have a peaked optical intensity output spectrum centred about a wavelength that corresponds with the pitch of the grating, and produce a peaked Raman amplification spectrum. The effective refractive index of the optical waveguide containing the grating may be tuned, thereby tuning the central wavelength of the reflection spectrum of the DBR, which governs the dominant output wavelength of the laser. However, the range of such wavelength tuning is limited to about 8-10 nm by material considerations, due to the maximum change in the effective refractive index that may be brought about in the material of the waveguide within the DBR section of the laser, and due to the need to allow for manufacturing variations. Accordingly, the output spectrum and Raman amplification spectrum of such DBR lasers may only be tuned over a limited wavelength range.
Due to their peaked Raman amplification spectra, neither a single DFB laser nor a single DBR laser is able to provide a Raman amplification spectrum that is tailored to a particular fibre-optic link, or is uniform. Accordingly, in wideband fibre-optic systems, in order to provide a more desirable amplification spectrum, it is necessary to multiplex a number of such DFB or DBR lasers having different operating wavelengths, which are operated at different intensities to provide an improved Raman amplification spectrum. For example, it is known to use an array of different DFB lasers or DBR lasers that are monolithically integrated and optically coupled through a common optical output.
However, with pump amplification systems having only a small number of multiplexed lasers, each having only a substantially fixed operating wavelength or a limited wavelength tuning range, the ability to optimise the Raman amplification spectrum is limited. Accordingly, to provide an enhanced amplification spectrum would require the multiplexing of a larger number of such lasers, which would increase the redundancy of components, inventory cost and space requirements within physically compact assemblies.
Further, when channels are added or dropped from use in a fibre-optic link, the Raman amplification spectrum of the other channels transmitted across the link will change. Existing Raman amplification systems, using fixed or limited tunability lasers, offer at most only low levels of re-configurability to optimise the Raman amplification spectrum following the adding or dropping of channels. Accordingly, the amplification performance of any particular channel varies, dependent upon which and how many other channels are being used.